This invention relates generally to free-tipped axial-flow fans, which may be used as automotive engine-cooling fans, among other uses.
Engine-cooling fans are used in automotive vehicles to move air through a set of heat exchangers which typically includes a radiator to cool an internal combustion engine, an air-conditioner condenser, and perhaps additional heat exchangers. These fans are generally enclosed by a shroud which serves to reduce recirculation and to direct air between the fan and the heat exchangers. Typically, these fans are powered by an electric motor which is mounted to the shroud.
The fans are typically injection-molded in plastic, a material with limited mechanical properties. Plastic fans exhibit creep deflection when subject to rotational and aerodynamic loading at high temperature. This deflection must be accounted for in the design process.
Although some engine-cooling fans have rotating tip bands connecting the tips of all the blades, many are free-tipped—i.e., the tips of the blades are free from connection with one another. Free-tipped fans have several advantages when compared to banded fans. They can have lower cost, reduced weight, better balance, and advantages due to their reduced inertia, such as lower couple imbalance, lower precession torque, and faster coast-down when de-powered.
Often free-tipped fans are designed to have a constant-radius tip shape, and to operate in a shroud barrel which is cylindrical in the area of closest clearance with the fan blades. In other cases, the tip radius is non-constant. For example, U.S. Pat. No. 6,595,744 describes a free-tipped engine-cooling fan in which the blade tips are shaped to conform to a flared shroud barrel.
Free-tipped fans are designed to have a tip gap, or running clearance, between the blade tips and the shroud barrel. This tip gap must be sufficient to allow for both manufacturing tolerances and the maximum deflection that may occur over the service life of the fan assembly. In practice, this gap is generally at least 0.5 percent, but less than 2 percent of the fan diameter, and more typically approximately 1 percent of fan diameter.
The presence of a tip gap has numerous adverse effects on performance. One effect is that as the gap increases the fan must operate at higher speeds to achieve a given operating point. This is due to the fact that the blade loading—the pressure differential between the pressure and suction sides of the fan blade—is reduced in the vicinity of the gap. Other effects are reduced fan efficiency and increased fan noise, particularly when the system resistance is high. These adverse effects can limit the applicability of free-tipped fans to applications where the system resistance is relatively low. Thus, there is a need for a free-tipped fan which minimizes adverse performance effects caused by the tip gap.
One approach is to design the fan so as to counteract the effect of the tip gap on the fan loading. For example, U.S. Patent Application Publication No. 2011/0211949 describes a fan with improved tip loading in the presence of a tip gap. This fan can improve fan performance, but the efficiency and noise of the fan may still be compromised by the gap.
Another approach is to design the tip of the fan in such a way that the flow of air through the gap is minimized. Various methods have been proposed in the past, with varying success. The challenge is to modify the blade shape in such a way that the flow through the tip gap is minimized, without adding geometric details which contribute additional parasitic drag or increase the noise of the fan.